analogWrite() avoids hostile takeover of PWM peripherals

Author: henrygab

cores/nRF5/wiring_analog.cpp

@@ -45,13 +45,30 @@
 extern "C"
 {
 
+class INTERNAL_ANALOG_STATE {
+  public:
+    uint8_t _lastAnalogWriteResolution = 8; // default value is 8-bit resolution (0..255)
+    uint8_t _moduleUsedByAnalogWrites[HWPWM_MODULE_NUM];
+    INTERNAL_ANALOG_STATE() : _lastAnalogWriteResolution(8) {
+      for (int i = 0; i < HWPWM_MODULE_NUM; i++) {
+        _moduleUsedByAnalogWrites[i] = 0;
+      }
+    }
+};
+
+static INTERNAL_ANALOG_STATE _AnalogState;
+
 /**
- * This will apply to all PWM Hardware
+ * This will apply to all PWM Hardware currently used by analogWrite(),
+ * and automatically apply to future calls to analogWrite().
  */
 void analogWriteResolution( uint8_t res )
 {
+  // save the resolution for when adding a new instance
+  _AnalogState._lastAnalogWriteResolution = res;
   for (int i = 0; i<HWPWM_MODULE_NUM; i++)
   {
+    if (!_AnalogState._moduleUsedByAnalogWrites[i]) continue;
     HwPWMx[i]->setResolution(res);
   }
 }
@@ -65,17 +82,50 @@ void analogWriteResolution( uint8_t res )
  */
 void analogWrite( uint32_t pin, uint32_t value )
 {
+  // If the pin is already in use for analogWrite, this should be fast
+  // If the pin is not already in use, then it's OK to take slightly more time to setup
+
+  // first attempt to find the pin in any of the HWPWM modules used for analog writes
   for(int i=0; i<HWPWM_MODULE_NUM; i++)
   {
-    // Added by if needed
-    if ( HwPWMx[i]->addPin(pin) )
-    {
-      HwPWMx[i]->writePin(pin, value);
-      return;
-    }
+    if (!_AnalogState._moduleUsedByAnalogWrites[i]) continue;
+    int const ch = HwPWMx[i]->pin2channel(pin);
+    if (ch < 0) continue; // pin not in use by the PWM instance
+    // already in use by this PWM instance as channel ch ...
+    HwPWMx[i]->writeChannel(ch, value);
+    return;
   }
-}
 
+  // Next try adding only to those modules that are already in use
+  for(int i=0; i<HWPWM_MODULE_NUM; i++)
+  {
+    if (!_AnalogState._moduleUsedByAnalogWrites[i]) continue;
+    if (!HwPWMx[i]->addPin(pin)) continue;
+    // added to an already-used PWM instance, so write the value
+    HwPWMx[i]->writePin(pin, value);
+    return;
+  }
+
+  // Attempt to acquire a new HwPWMx instance ... but only where
+  // 1. it's not one already used for analog, and
+  // 2. it currently has no pins in use.
+  for(int i=0; i<HWPWM_MODULE_NUM; i++)
+  {
+    if (_AnalogState._moduleUsedByAnalogWrites[i]) continue;
+    if (HwPWMx[i]->usedChannelCount() != 0) continue;
+    // added to an already-used PWM instance, so write the value
+    HwPWMx[i]->setResolution(_AnalogState._lastAnalogWriteResolution);
+    HwPWMx[i]->writePin(pin, value);
+    return;
+  }
+
+  // failed to allocate a HwPWM instance.
+  // output appropriate debug message.
+  LOG_LV1("ANA", "Unable to find a free PWM peripheral");
+  // TODO: Add additional diagnostics to function at higher log levels
+  return;
 }
 
+} // end extern "C"
+